1. Field of the Invention
This invention reIates to magnetic data storage devices, and particularly to equalization of data signals read back from magnetic data storage devices.
2. Description of the Prior Art
References:
Aaron, M. R. and Tufts, D. W., "Intersymbol Interference and Error Probability." IEEE Trans. Inform. Theory, IT-12 (1), 26-34 1966 (hereinafter, "Aaron"). Kallmann, H. E., "Transversal Filters." Proc. IRE, 28, 302-310, July 1940 (hereinafter, "Kallman"). Lucky, R. W., "Automatic Equalization for Digital Communications." Bell Systems Tech. J., 45(2), 255-286 1966 (hereinafter, "Lucky"). Proakis, J. G., Digital Communications. New York: McGraw-Hill Book Company, 1983 (hereinafter, "Proakis").
The readback signal from magnetic recording heads, such as those used in disk drives, has a characteristic response having the form of a pulse for every magnetization reversal (transition) on the recording media. Information in a disc drive is typically stored as the time spacing between pulses or magnetic transitions. The minimum time spacing between transitions is determined by the "width" (the extent to which a pulse can interfere with its neighbors) of the readback pulse. If the spacing is too small, adjacent pulse interference distorts the time spacing. This can cause errors in the information read back. Thus, the pulse width limits the information density and capacity of the disc drive.
If the width of the readback pulse can be minimized, the information density can be increased. Unfortunately, minimizing the pulse width increases the noise. This noise increase can also increase the chances of making an error. For a given probability of error and information density, there is an optimum trade-off between errors from noise and errors from pulse interaction (see Aaron). This optimum trade-off can be viewed as a particular pulse response or width. A filter that transforms the given readback signal into a signal that has the desired trade-off is normally called an equalizer.
In disc drives, the magnetic transitions are stored in concentric circles known as "tracks". The recording head is mechanically moved to the desired track and information can be stored and retrieved. To simplify the electronics, the information content of each track is kept constant over the whole media surface or part of the surface (constant zones). Because the Circumference of the tracks varies, and the timing is constant, the width of the pulses changes from one track to another. In addition, the recording head typically flies on an air cushion (hard discs) to minimize wear. The air cushion thickness varies with tangential veIocity, which is a function of track location. This results in an additional variation of pulse width with track location. This pulse width variation preclides the use of a constant or fixed value fiIter (equalizer) for optimizing the pulse width for all tracks.